Localization system



v p 1, 1953 E. L. G. TORCHEUX ET AL 2,651,032

LOCALIZATION SYSTEM Filed March 1, 1949 7 Sheets-Sheet 5 m w 0W v Am we. m aw 5 0 mm M V ww MA .5 $7 v/ WW3 fi Patented Sept. 1 i953 BFE'EQE LOCALIZATION SYSTEM Emile-Leon Gabriel Torcheux and Etienne Augustin Henri Honor, Paris, France Application March 1, 1949, Serial No. 79,074 In France March 2, 1948 2.6 Claims. (Cl. 343-105) This invention relates. to a location system and more particularly to a new system for the location of a receiving point of unknown position relative to a plurality o ftransmitters of known position by comparing at said receiving point the phases of signals emitted by the transmitters.

As i l nown, such comp r son a e it possible to determine .eqi iphase lines passing through the receiving point, the position of which is to be ascertained and this way enable the location thereof to be determined.

n khewh y ms, th s gna s emit by he. transmitters must stay in rigid relation to each other and generally they are the harmonics of a single q ne Th s has a y d a s- Tlee d de he atus is ahnl ee ed and the utilization of harmonic freguencies involves the necessity f e hle hs hume e s freq e y bands. Also the elimination of the ambiguities which are inherent to all eeuiphase line systems, involves considerable difiiculties, especially if a complete elimination of ,these ambiguities is desired.

In accordance with the present invention, practies-11y y uen may e uti zed. I is moreover possible to employ a lesser number of wave lengths than in prior systems. Also, the elimination of the above-referred to ambiguities is very simply accomplished withrelatively simple apparatus.

It has been foundthat itis always possible to obtain from at least three signals having frequencies F1 F2, F, by operations such as frequency mixin irequehey multip yi filtering, etc., or combinations thereof, at least one pair of derived signals having frequencies Fe and Fe such that the following relation is satisfied:

FaF'"b=K1 F1+K2F2 +KnFn where K1, K2, K are integers, at least one of which is positive and one of which is negative.

If the frequencies Fr, E2, Fn and the integers K1, IQ, Kn are seleetedsuch that KlFl-i-KZFZ then FcL- Fb- '.0 or EQFFI? Thus two signals, of exactly egual frequency can be obtained which can be comparedand their phase difference s?a qb determined by a phasemeter.

The following relation can be derived from the first relation above givenj where 1, gig and es are the respective phases of the varieii si na an .G- i a c n n w i can be made zero by adjustin a hase i h syste above r a eh a e valid at any p nt i a sys mi the si nals hav ng the f eq en ie F1 Fa, Ft e ra smi ted throu h a e f m at least two spa e transmi i g leeihts ar divided etween at l ast twe space transmitters a ece ve at a s n le reeeivih poi h following re a ions ich can be d rive r m e a elai ab v iven:

where D1, D2, D1, are the respective distances between the receiver and the transmitters emitting .the signals F1, F2 F11, V is the velocity of propagation of the signals through space, .s, the and b are in radians and refer to the phase relations of the .two signals of equal frequency derived at the receiver, and C is a constant, which if desired can he made zero so that for practical purposes the last relation can be written as follows:

From either of the last two relations, plots of equiiehase lines in sp e can be pr p r f any system of any number of spaced transmitters r ate th n n which. transmits y number of frequencies, greater tha two divided between the transmitters If K1, K2 Kn and F1, F2 F11 are selected Q1 known such that the relations K F1+K2F +KnF11=0 is satisfied, two sigia s of exactly equal frequency can b obtained at the receiving point so that s can be directly a ulated or m asur last relation for (15s given above may be r tten in th form In this relation-Fe is a number which may be termed the characteristic frequency of the type of system thus far discussed. It defines the density of the equiphas'e'lines provided by such a system.

Also in this relation, 1 (D) is a linear and simple function of the distances D1, D2 D11. between the various transmitters of such a system and the receiving point. 'This function defines the shape of the equiphaselines.

From the relation K1F1+K2Fz +KnFn=0,

it follows that all of the frequencies of the transmitted signals can vary within limits provided that at least one of the frequencies is compensatingly varied to maintain the value of zero. The variation of the various frequencies merely affects the accuracy of the distance determinations, and if the percentage variations of the frequencies are small, the percentage error in the distance determinations is correspondingly small. That is to say, all of the transmitters may have their frequencies determined, for example, by independent crystal-controlled oscillators and the oscillator for one of the stationsalso controlled from information obtained from signals received from all of the transmitters to compensatingly vary the frequency of the one station.

Also from the relation it follows that the phases of the various signals from the transmitters may vary within limits and the phase of at least one of the transmitted signals compensatingly varied from information obtained from signals received from all of the transmitters so as to maintain C constant or zero. Thus all of the transmitters except one may be independent of each other provided the frequency and phase of the signal from one of the transmitters is varied to compensate for frequency and phase drift in the other transmitters. This can be easily accomplished automatically by providing a receiving station at a known location with respect to the transmitters to receive the signals from all of the transmitters and feeding back the required correcting information from this receiving station to the one transmitter.

The system so far described may be termed an elementary system. The information derived at the receiver merely enables the receiver to be located on a line of given phase shift ([55. In any elementary system there will ordinarily be a plurality of lines of the same phase shift so that an ambiguity exists as to which line of this phase shift passes through the receiving point.

By employing a plurality of elementary systems combined in what may be termed a multiple system so as to produce a plurality of sets of equiphase lines all having the same shape but different spacings between the lines of equal phase shift, this ambiguity can be eliminated. For example, the sets of equiphase lines can have relative spacings between lines of equal phase shift which differ from each other by a factor of 10, i. e., are proportional to 1, 10, 100 etc. By measuring at a receiving point of unknown location the phase differences between the two derived signals of equal frequency corresponding to each set of equiphase lines, it can be determined which line of equal phase shift passes through that receiving point. Such a multiple system, however, locates the receiving point on a line of equal phase shift only and does not locate the position of the receiving point on that line.

By employing a combination of two or more multiple systems in what may be termed a complete system to provide a plurality of families of intersecting equiphase lines, it is possible to locate the receiving point on two or more lines intersecting at a point to thus determine the position of the receiving point without ambiguity.

The invention also contemplates more elaborate systems which may be termed combined systems. Such systems are a combination of a plurality of complete systems and may be arranged so that the frequencies employed are con- 4 centrated in a few relatively narrow frequency bands, and a single receiver may be adjusted to receive and utilize the frequencies corresponding to any desired elementary or multiple system.

An object of the present invention is therefore to provide elementary, multiple, complete and combined location systems operating in accordance with the'principle above discussed.

Other objects and advantages of the invention will appear in the following description of vari-'- ous modifications thereof shown in the attached drawings, of which:

Fig. 1 is a block diagram of an elementary sys*- term in accordance with the present invention employing five frequencies.

Fig. 2 is a block diagram showing a modification of the transmitting station of Fig. 1 illustrating how one of the transmitters may be controlled by signals received by wire from all of the transmitters in order to compensate for drift in frequency and phase by the various transmitters.

Fig. 3 is a block diagram of the transmitting station of Fig. 1 showing in more detail the mechanism for controlling one of the transmitters by control signals received from a control receiving station in order to compensate for drift in frequency and phase by the various transmitters.

Fig. 4 is a block diagram of a receiver suitable for employment in the system of Fig. 1.

Fig. 5 is a diagram similar to Fig. 1 showing a modified elementary system employing four frequencies.

Fig. 6 is a block diagram of a portion of a receiver in accordance with the present invention Which may be termed a subtraction filter.

Fig. 7 is a block diagram of a modified receiver suitable for employment in the system of Fig. 5.

Figs. 8, 9 and 10 are block diagrams of modified receivers suitable for systems employing four frequencies.

Figs. 11, 12, 13 and 14 are plots showing different patterns of equiphase lines obtained with ele mentary systems employing four signals.

Fig. 15 is a block diagram of receiver suitable for employment in a long-range combined system.

Fig. 16 is a block diagram of a transmitter suitable for employment with the receiver of Fig. 15 in a long-range combined system.

Fig. 17 is a map showing an example of suitable locations of the transmitting stations in a longrange combined system.

Fig. 18 is a block diagram of a receiver suitable for employement in a short-range combined systern.

Fig. 19 is a block diagram showing two transmitters suitable for employment with the receiver of Fig. 18 in a short-range combined system; and

Fig. 20 is a map showing an example of suitable locations of the transmitting stations in a shortrange combined system.

Referring to Fig. 1 of the drawings, this figure shows, by way of example, a location system including a transmitting station having five frequency generators, l to 5 inclusive, each gener- 5 described. The aerials A to. D inclusive may, for example, be arranged at the four cornersof a. square. inscribed in a. circle of a radius of one. kilometer at the center of which aerial E is. positioned.

The. frequencies of the transmitted signals may, for example. have the following values respectively:

1,499.,i7'0. cycles (A), 1,500,500 cycles (B), 1,500,390 cycles 1,499,700 cycles (D) and 1,560,000 cycles (E1).

The transmitting station may include a control or regulation receiver provided with an aerial Ry. As will be described hereinafter this receiver is used to control or regulate the generator which feeds the aerial E. The location of Re is fixed and its distance from aerial E may, forexample, be 2,400 meters.

The system of Fig. 1 may also include a mobile receiver H having a receiving aerial R with respect to which it is desired to determine the distance to the transmitting aerial E. As will be explained below the mobile receiver may be employed to derive two low frequency currents from the currents resulting from the received signals and to measure the relative phase shift 1155 of said currents.

This relative phase shift is. related, to the position of the mobile receiver and depends on the distance D of the mobile receiver from transmitting aerial E. With the frequencies and arrangement of transmitting. aerials. above assumed, when the distance D is several kilometers it can be shown that approximately $5 being expressed in degrees and D in kilometers. Thus at. a distance D greater than a few kilometers; the equiphase lines are approximately concentric circles having centers at the location of the. transmitting aerial E.

It is to be noted, that a knowledge of the valueof distance I) alone is not sufiicient to completely determine the position, of the mobile receiver and that if distance D becomes smaller than 5 kilometers the phase shift exceeds 360 degrees. That is to say the relative phase shift 5 is the same as if D were very great. As it is well known, such ambiguities occur, in mostlocalization systems which use-phase shift measurements.

It will. be shown below how more complete systems make it possible to eliminate both these ambiguities.

The mobile receiver ll of Fig. 1 may include a circuit i2 shown in more detail in Fig. 4, which circuit may be termed a frequency operator and may also include a phasemeter- [3; to indicate the relative phase shift s of the two currents of equal: frequency referred to above, which are derived by the frequency operator [2 from the five signals of difierent frequencies radiated from the aerials A to E and received by the receiving aerial R.

It is to be noted that the transmitting station also includes a frequency operator II as part of its controli or regulation receiver 5. In the system of Fig. 1, the frequency operator l2 of the transmitting station receives signal energy from all of the transmitting aerials A to E through the aerial Rr. However; as shown in Fig. 2, it is entirely possible to supply signal energy-tothefrequency operator [2 of thetransmitting station by direct wires from the connections it to Zll' which connect the generators l to 5 respectively to the amplifiers & to I'll respec tively.

As shown in Fig. 2, the. synchronizing generator 5 may include a frequency generator is labeled GE, which may be the same as the generators l to 4 except that its frequency may be controlled or regulated to compensate for frequency drift in the various generators including generators i to 5 inclusive. It may also be provided with a phase shifter I5 in the connection 26 between the generator M and the. amplifier it. A two-phase asynchronous servo-motor 2i, having a shaft 22 driving a shaft 23.- preferably through a gear reduction mechanism (not shown), may operate the phase shifter 15 to shift th phase of the signal supplied fromthe generator i i to. the amplifier Hi. The. motor 25 may also drive a shaft 24 which, for example, mechanically varies an element of the frequency-determining circuit, such as a variable condenser (not shown)- in the generator- I l, to vary the frequency generated thereby. As stated. above, the frequency operator l2 derives two currents from thesignals supplied thereto from generators l.- to 5 inclusive. As long as these two currents are. equal. in phase and frequency, 1. e., as longas the above-given relations are satisfied, at a predetermined point in thesystem so as toproduce the two currents of equal phase and frequency, the shaft 22 of the motor 2! does not rotate. When, however, these relations are not satisfied because of a phase or frequency drift inany of the generators, the rotor of the motor 21 will rotate and the mechanical connections to the generator M and the phase shifter 55 can be arranged to vary the frequency and phase of the generator 5 to again satisfy the above relations. The shaft of the motor then stops. The frequency operator E2 of Fig. 2 and synchronizing generator 5 thus constitute a feed-back loop or follow-up mechanism which keeps the system constantly in synchrcnism so as to satisfy the above relations. This is accomplished by adjusting the phase or frequency or both of one generator and maintains the two currents. derived from the five generated frequencies by a,- frequency operator anywhere in the system at the same frequency.

The system of Figs. 1 and 3 is similar to the system of Fig. 2, except that the signal energy supplied to the frequency operator i2 is received by the receiving aerial R1 from the transmitting aerials A to E inclusive by transmission of radio waves instead of through wires, and the two currents derived from the signals by the frequency operator I2 are supplied by wires 26 to the servo-motor 2|. In Fig. 3', one of the generators I to 4 ofFig. 1' and one of the amplifiers 5 to 9 respectively of Fig. 13 formpart of each of the transmitters labeled TR, which feed the transmitting aerials A to D.

Fig. i is a block diagram showing the mobile receiver of Fig. 1 in greater detail. This receiver includes a frequency operator l2 which derives the two currents having the frequencies Fa and. Pb referred to above.

The frequency operator includes an amplifying filter 30 labeled AF which receives. the signals 75. from the receiving aerial R.

For the frequencies,

FA=1,499,470 cycles FB::1,500,500 cycles Fc:1,500,330 cycles Fp=1,499,700 cycles, and FE:1,500,000 cycles which were referred to above and which may be radiated from the transmitting aerials A to E respectively, this amplifying filter may pass a band of 15 and kilocycles so as to pass the frequencies just mentioned. The receiver may also include an oscillator 3| labeled HE which supplies a heterodyn frequency of 1,972 kilocycles. The frequency operator l2 also includes a mixer 32 labeled MI to which is delivered all of the frequencies passed by the amplifying filter 30 and also the heterodyne frequency from the oscillator 3:. The output of the mixer 32 may be delivered to an amplifying filter 33 which may pass a band of frequencies of 472:2 kilocycles. This amplifying filter 33 passes only the difference frequencies between the heterodyne frequency of 1,972 kilocycles and the frequencies radiated from the aerials A to E and passed by the amplifying filter 30. That is to say, the output of the amplifying filter 30 contains the following frequencies: 472,530; 471,500; 471,670; 472,300and 472,000 cycles. These frequencies are delivered to another mixer 34 and the output thereof delivered to two amplifying filters 35 and 36. Amplifying filter 35 passes a band of 515:50 cycles. That is, it passes the difference frequencies between 472,000 cycles on the one hand and 472,530 and 471,500 cycles on the other hand, which difference frequencies are 530 and 500 cycles respectively. The amplifying filter 36 passes a band of 315:50 cycles. That is, it passes the difference frequencies between 472,000 cycles on the one hand and 471,670 and 472,300 on the other hand, which difference frequencies are 330 and 300 cycles respectively.

The output from the amplifying filter 35 is delivered to another mixer 3! and the output of the amplifying filter 36 is delivered to another mixer 38. The output of the mixer 31 is delivered to an amplifying filter 39 which passes a band of 30:20 cycles. That is, it passes the difference frequency between 530 and 500 cycles, or 30 cycles. Similarly, the amplifying filter '40 passes a band of 30:20 cycles. That is, it passes a difference frequency between 330 and 300 cycles, or 30 cycles. The outputs of amplifying filters 39 and 40 are the two currents Of equal frequency Fa and Pb,

The mobile receiver also includes a phase= meter 13 by which the relative phase (Ps of the two currents Fe, and Fb of equal frequency, name- 1y, 30 cycles, is indicated.

That the phase difference S between these two currents is a function of the frequencies transmitted and received, the arrangement of the transmitting aerials and the distances of the mobile receiver from the transmitting aerials is easily shown.

The five transmitted frequencies given above respond to the relation K1F1+K2F2 KnFn O. That is to say, if the frequencies transmitted from the aerials A to D inclusive are each multiplied by l and the frequency transmitted by the aerial E is multiplied by +4, the relation is satisfied, In this specific example, the frequency of 1,500,000 cycles transmitted by the aerial E is the average of the other four transmitted frewill also be true that K1F1+K2F2 +KnFn:2 such that the relation Fs.Fb:KlFl+K2F2 KnFn is still satisfied. This is a general proposition which will always be found true of the frequencies employed in the present invention. The value of both sides of the last-written expression can be made zero, for example, by compensatingly varying the frequency of one of the generators such as by increasing the frequency transmitted by the aerial A by one-half cycle, i. e.. to 1,500,000.5 cycles.

The relation can be derived from the relation as follows.

The frequency F of any signal is the rate of change with respect to time of the phase 5 of that signal. That is to say or it can be generally stated that where C is a constant associated with the apparatus employed.

Integrating both sides of (2) in accordance with (3) ives This relation in common with (2) and (3) is currents of equal frequency and the phase of relation between the phases of the two derived valid anywhere in the system and gives the the signals.

At the transmitters 4m are the phases of the signals at the transmitters and Ce is a constant associated with the transmitters. The phase difference a-b can be made zero at any point in the system, for example, a point at or near one of the transmitters, by adjusting one of the phases 1e, 2e me such that Relation 4 is also valid at the receiving point such that at the receiver where 1r, 2r 1uare the phases of the various signals at the receiving point and C1 is a constant associated with the receiver.

However,

( ..b= ;(K1F1D1+K2F2D2 +K..F.D. +c

That is to say, the phase diiference s:q5a'-b between the two derived currents of equal frequency at the receiver is a function of the transmitted frequencies and the distances of the receiver from the transmitting aerials. The constant C represents a phase and can be made zero or given any desired value by adjusting the phase of any signal in the system, for example, by adjusting the phase shifter l5.

t is known that a radio signal having a frequency of 1,500,000 cycles travelling through a distance D expressed in kilometers undergoes a negative phase shift equal in degrees to -1,800 D. Thus, in the present example the relative phase shift 4n expressed in degrees will be given approximately by the expression:

That is to say,

and the equiphase lines are approximately concentric circles having centers at the location of the transmitting aerial E.

Fig. 5 is a block diagram of an elementary system employing four signals radiated from four aerials A, B, C and D. The transmitting station, labeled MG, includes a synchronizing generator l' which may be similar to the synchronizing generator 5 of Figs. 1 and 3. It also includes three independent generators 2', 3' and 4', the generators l to 4' inclusive feeding the aerials A to D respectively. The transmitting station is also provided with a receiver having an aerial RI of fixed location and may have a frequency operator 42. The system of Fig. 5 also has a mobile receiver labeled MP and having an aerial R and a frequency operator 42 which may be the same as the frequency operator 42 of the receiver of fixed location. These frequency 0perators include two circuits 43 and 44 labeled SF which may be termed subtraction filters. As shown in Fig. 6, each subtraction filter may include an amplifying filter 45 and a mixer 46. In each instance, the two subtraction filters 43 and 44 have signals delivered thereto from a receiving aerial.

The frequency operator 42 is also provided with two amplifying filters 41 and 48 to which are delivered the outputs of the subtraction filters 43 and 44 respectively. The outputs of the amplifying filters 4! and 48 are two currents of equal frequency when the system is in synchronism. In the case of the mobile receiver, the two currents are fed to a phasemeter 49 which indicates the diiference in phase between the two currents. In the case of the receiver of fixed location, the two currents are fed by the lines 5| to the synchronizing generator I in order to insure that the relations are satisfied as explained with reference to Figs. 1 and 4.

As a specific example, the signals radiated from the aerials A to D respectively and the corresponding values of K may be as follows:

F =520,020 K=+1 F2=520,O00 K=1 All of these signals are received by the aerial R of the mobile receiver. The amplifying filter 45 (Fig. 6) of the subtraction filter 43 (Fig. 5) passes the frequencies F1 and F2 and these are mixed in the mixer 46 (Fig. 6). The amplifying filter '41 (Fig. 5) passes the difierence frequency of 20 cycles. Similarly, the subtraction filter 44 passes and mixes the frequencies F3 and F4 and the amplifying filter 48 passes the resulting difference frequency of 20 cycles. The resulting two currents, each of 20 cycles, are delivered to the phasemeter 49 and the phase difference (#5 indicated thereby. When es is expressed in degrees,

where D1 to D4 are the distances between the aerial R of the mobile receiver and the transmitting aerials A to D respectively. The above relation may be put in the approximate form where Fe is the characteristic frequency of the system and is approximately 500 kilocycles. This makes the value of the coefficient equal to 0.6 per meter.

The frequency operator 43 of thetransmitting station operates in the same manner as that of the mobile receiver and, as stated above, furnishes two currents to the synchronizing generator I to maintain the system in synchronism.

Fig. 7 illustrates another type of receiver, the frequency operator circuits of which may be employed for either the fixed receiver or the mobile receiver of Fig. 5. In this figure, the signals received by a receiving aerial from the transmitting aerials are delivered through a line 50 to a mixer 53. A heterodyne frequency from an oscillator 52 is also delivered to the mixer 53. The output of the mixer 53 is delivered to two subtraction filters 43 and 44' of the general type illustrated in Fig. 6, and the outputs of these subtraction filters are delivered to the amplifying filters 4'! and 48 respectively which in turn deliver two currents of equal frequency to the phasemeter 49.

As a specific example, the four frequencies radiated by the four transmitting aerials and their corresponding values of K may be as follows:

F1=1,000,320 cycles K1=+1 F2=1,000,300 cycles K2=1 F3=1,000,220 cycles K3=--1 F4=1,000,200 cycles K4=+1 The frequency H generated by the oscillator 52, may be 1 megacycle.

The mixer 53 performs the following subtractions of frequencies:

and produces, therefore, currents having the following frequencies respectively:

The subtraction filter 43' selects the frequencies f1 and f2 and mixes these frequencies to produce a difference frequency of 20 cycles so as to deliver a current of this frequency to the phasemeter 49. The subtraction filter 44 selects the frequencies is and f4 and mixes these frequencies. The amplifying filter 48 passes the difference frequency of 20 cycles and delivers another current of this frequency to the phasemeter 49.

In the above-described systems employing four signals, the signal frequencies have been composed of pairs of frequencies in which the frequencies of each pair have been close to each other but in which the average frequencies of the pairs of frequencies are quite different. The receiver of Fig. 8 is adapted to operate with signals having frequencies which need not have the above-mentioned relation to each other. In accordance with Fig. 8, signals of four different frequencies may be received by four receiving aerials and delivered through the lines 54, 54 55*- and 55 respectively. A heterodyne frequency may also be delivered to the mixers 5! and 58 from the oscillator 56 and, similarly, a heterodyne frequency may be delivered to the mixers 57 and 58 from an oscillator 56 The two heterodyne frequencies from the oscillators 56 and 56 may be different frequencies, although in some instances they may be the same frequency, in which case a single oscillator may be employed. The outputs of the mixers 5'! and 51 may be delivered to a subtraction filter 59, which passes and mixes two frequencies from the mixers 5'! and 51 Similarly, the outputs from the receivers 58 and 58 are delivered 0 a subtraction lected to have the same frequency if the relation.

of the received signals satisfies the relation F1K1+F2K2+F3K3+F4K =0 In this particular case, the values of K for two of the frequencies will be +1 and for the other two frequencies will be 1.

As a specific example, the values of the various transmitted frequencies and their'corresponding. values of K may be The values of the heterodyne frequencies He. and Hb from the oscillators 56 and 56 may be Ha=97,5 kc.+10 cycles Hb=99,5 kc.-10 cycles Mixer 5'! produces'a frequency which is the sum of Ha and F1 or 115,010 cycles. Mixer 51b produces a frequency which is the sum of Hb and F3 or 114,990 cycles. These two frequencies are selected and mixed by the subtraction filter 59 to produce a difference frequency of 20 cycles, which is passed by the amplifying filter 60, and a current of this frequency delivered to the phasemeter B3.

Mixer 58 produces a frequency which is the sum of Ha and F2 or 112,010 cycles. Mixer 58b produces a frequency which is the sum of He and F4 or 111,990 cycles. These two frequencies are selected and mixed by the subtraction filter Bl to produce a difference frequency of 20 cycles. This frequency is passed by the amplifying filter 62 to deliver a current of this frequency to the phasemeter 63.

Fig. 9 differs from Fig. 8 only in that a servomotor 64 is substituted for the phasemeter 63 and through a mechanical connection indicated at 65 the servo-motor drives a phase shifter 65 positioned in the linedelivering the heterodyne frequency from the oscillator 53' to the mixer 58 The mechanical connection preferably includes a gear reduction mechanism (not shown).

The receiver of Fig. 9 operates as a follow-up system. The shaft of the motor 64 takes a position which is such that the phase difference between the currents which feed the servo-motor 64 is zero and this position indicates the value of the phase difference.

It is to be noted that the phase shifter 66 could be interposed in other circuits, for example, any one of the input circuits of the devices 51 51 58 58 59 and 6|, but it is generally of advantage to act on a locally generated current since its frequency may be made stable and free of harmonics.

The phase shifter 66 and the servo-motor 64 may be similar to the servo-motor and phase shifter described with reference to Figs. 2 and 3.

Fig. 10 shows a receiver which is similar to that of Fig. 8. In this figure the mixer 61* replaces the mixers 5'! and 58 of Fig. 8 and the mixer 61 replaces the mixers 51 and 58 of Fig. 8. All of the signals from a single aerial are delivered by a line 68 to both the mixers 6'1 and 61 instead of being delivered separately to eachv of the mixers as in Fig. 8. Assuming the same frequencies given in the specific example discussed with respect to Fig. 8, the mixer .61 will now produce the two frequencies of 115,010 and 112,010 produced in the two mixers 51 and 58, and the mixer fil will now produce the two frequencies of 114,990 and 111,990 produced in the two mixers El and 58, respectively, of Fig- 8. The subtraction filter 59 of Fig. 10 will select the two frequencies of 115,010 and 114,990 and mix them to produce a difference frequency of 20' cycles in the same manner as the subtraction filter 50' of Fig. 8. Also the subtraction filter 6| of Fig. 10 will select the. two frequencies of 112,010 and 111,990 and mix them to produce a difference frequency of 20 cycles, in the same manner this is accomplished by the subtraction filter SI of Fig. 8. Otherwise the receivers of Figs. 8 andld are the same and operate in the same manner.

Fig. 11 gives an example of the pattern of equiphase lines which may be obtained from the transmitting station of Fig. 5. In Fig. 11, A, B, C, D, Br and R designate the supposed locations of the various aerials. It is assumed that the frequencies F1, F2, F3 and F4: supplied by the generators l, 2, 3' and 4 respectively and the corresponding values of K are:

in this case the phase difference between the two currents derived from these signals is where Dr to D4 and the respective distances from the transmitting aerials to the mobile receiver. This may be expressed as follows."

such that the characteristic frequency is 510,000 cycles.

Fig. 12 gives an example of another pattern of equiphase lines which may be obtained from the transmitter of Fig. transmitting the same frequencies, if the aerial C is omitted and the aerial B is fed by both the generators 2" and 3". this case the three transmitting aerials are positioned in a straight line with the aerial B" located half way between the aerials A and D.

Fig. 13 shows an example of the equiphase lines obtained from the transmitter of Fig. 5 if the aerials C and D are omitted, the aerial A feed from both the generators l and 4' and the aerial B fed from both the generators 2' and 3. In this case, D1 equals D and D2 equals D3 and the approximate value of as (assuming the same values of transmitted frequencies assumed with reference to Fig. 11) is as follows:

i4 and D2 equals D4 and the approximate value of s is a -20,000 (D -D such that the characteristic frequency is 20,000 cycles.

According to a last modification of the system shown in Fig. 5 the aerial D is omitted; the aerial A is fed by the generator I'and thus sends out the signal ofv frequency F1; the aerial B is fed by the generator 2' and thus sends out the signal of frequency F: and lastly, the aerial C fed by both generators 3 and 4- and thus sends out both signals of frequencies F3 and P4.

In this case Dz equals D4 and the approximate value of 95s is such that the characteristic frequency is 520,000 cycles. The distances D3 and Dr do not appear in this approximate expression, which means that the aerial transmitting the signals of the frequencies F3 and F4 can occupy any position, for example, the position of either of the aerials A or B. From the above. it is apparent that the characteristic frequency of a system depends upon the number of transmitting aerials employed to transmit the various signals.

Multiple and complete systems As. stated above, the invention contemplates multiple systems made up of a plurality of elementary systems which provide a plurality of patterns of equiphase lines of the same shape but having different spacings between the lines. Thus, for example, any number of such elementary systems in which the spacings. between equiphase lines are proportional to 1, 10, 100, etc., may be employed to resolve the ambiguity as to which of the equiphase lines passes through aerial of the mobile receiver.

The invention also contemplates the employment of a plurality of elementary systems or a plurality of multiple systems to provide a plurality of patterns of intersecting equiphase lines in order to resolve the ambiguity as to the position of the aerial of the mobile receiver on a given. equiphase line.

A. specific example of a system involving two multiple systems to produce a complete system may be as follows:

The transmitting station may include three spaced aerials A, B and C arranged so that the signals from the aerials A and B provide three patterns of superimposed, parallel equiphase lines in which the spacings between the lines in the second and third patterns are respectively and /104; of those of the first pattern. Also the signals from the-aerials A and (1 provide three similar patterns of equiphase lines intersecting those from the aerials A and B.

According to this example, 12 generators prodoes 12 having the following frequencies respectively:

Al=l6,0QO c. FA2=I4,000 c. FA3=13,000 c. FA4=15,700 C.

1 31 15300 C. FBs=14.2fi0 2.

The signals having the frequencies indicated by the subscript A in the above list are radiated from the aerial A and, similarly, the frequencies 15 indicated by the subscripts B and C are radiated from the aerials B and C respectively.

From the above list of frequencies, six groups of frequencies, all of which satisfy the relation may be selected. This relation may be maintained by regulating one or more of the generators supplying a frequency in each group in accordance with the principles discussed above Two currents of equal frequency can be derived from each group of currents and their phase differences determined.

One group G1 formed of the four currents havin the frequencies F111, FAZ, F131, Fez may have values of K of +1, +1, +1, 1 respectively, theabove relation being maintained by regulation of the generator supplying the current of frequency F132.

A second group G2 formed of the four currents: having the frequencies FAI, FA3, F131, F133 may have values of K of +1, 1, +1, +1 respectively, the above relation being maintained by regulation of the generater supplying the current of frequency F133.

A third group G3 formed of the four currents havin the frequencies FAl, FM, Fm, F134 having values of K of +1, 1, -1, +1 respectively, the above relation being maintained by regulation of the generator supplying the current of frequency F134.

A fourth group G4 formed of the four currents having the frequencies Fm, F112, F01, F02 having values of K of +1, +1, -1, 1 respectively, the above relation being maintained by regulation of the generator supplying the current of frequency F02.

A fifth group G5 formed of the four currents having the frequencies Fm, FAa, F01, F03 having values of K of +1, 1, +1, +1 respectively, the above relation being maintained by regulation of the generator supplying the current of frequency F03.

A sixth group Gs formed of the four currents having the frequencies Fm, FAA, F01, F04 having values of K of +1, 1, 1, +1 respectively, the above relation being maintained by regulation of the generator supplying the current of frequency F04.

The mobile receiving station receivers include frequencyo-perators deriving two currents of equal frequency from each of the siX groups of signals G1 to G6 and measure their phase difierences to obtain (p51 to s6. These phase differences are related to the position of the mobile receiver approximately as follows:

DA, DE, D0 being the distances expressed in kilometres which separate the mobile receiver from the transmitting aerials A, B, C respectively and the values the being expressed in degrees.

The system just described, therefore, is a complete system which includes six elementary systems divided into two multiple systems, one of which provides three determinations of different magnitude of the distance of the mobile receiver from the aerials A and B and the other of which provides three determinations of different magnitude of the distance of the mobile receiver from the aerials A and C. The position of the mobile receiver can thus be determined with accuracy and without ambiguity.

' It is to be noted that the six groups G1 to G6 of four signals each are obtained from twelve signals instead of twenty-four thus enabling less apparatus and a narrower frequency band to be employed.

It will also be noted that the frequencies of the signals transmitted from aerial A satisfy the following relations:

FA1+FA2 =3o,00o cycles FA1FA3=3,O00 cycles FA1+FA4=300 cycles The same is true of the signals transmitted .from aerial B and of the signals transmitted from aerial C. These are the characteristic frequencies associated with the three elementary systems respectively of each multiple system of the present example.

It is to be noted that three additional groups of four signals satisfying the relation can be obtained from the above list of twelve signals. Such groups of signals are as follows:

Group G7FB1, F32, F01, F02 Group G8FB1, F133. F01, F03 Group G9FB1, F134, F01, F04

From these additional groups another set of three elementary systems giving three patterns of equiphase lines of different spacings intersecting the other two patterns may be obtained, if desired. It is to be noted that no further regulat1on of any of the frequency generators will be required as two of the generators furnishing a frequency in each of the three additional groups are already subject to regulation.

Application to the long range navigation The complete system shown in Figs. 15 to 1'7 may include 25 stationary transmitting stations which are called respectively station A, station B, station Z omitting one letter of the alphabet, for example, J, and each of which comprises an aerial called aerial A, aerial B aerial Z respectively. The variou transmitting stations may be positioned at considerable distances from each other, for example, they may be distributed over a wide area as shown in Fig. 17 which is a map of the Atlantic Ocean indicating appropriate locations for ten transmitting stations, the remaining 15 being positioned in other parts of the world.

Each of these aerials transmits in the form of four pure undamped waves four signals.

The system is based on four basic frequencies, F1, F2, F3 and F4 which have the following relation to each other:

F1+F2=30,00O cycles F1F3=3000 cycles F1F4=300 cycles In the present example, the following basic frequencies have been chosen:

F1 =15,030 c. F2=14,970 c. F3=12,030 c. F4=14,730 c.

The four signals transmitted b y station A are derived from the basic frequencies as follows:

l? Similarly, the frequencies of the signals transmitted from station B are derived as follows:

FB1=F1+fB FB2=F2'fB FBs=F3+fB FB4=F4+fB .A similar derivation is employed for the remaining stations. Also the factors fa to in were chosen as follows:

f1t=120 c. 3:240 c. fc=360 c.

Thus station A transmits signals having the following frequencies:

FA1=15,O30+120=15,15O cycles. FA2=14,970120=14,850 cycles FAe=12,03D+120-=12,150 cycles F.44=l4,730+120=14,850 cycles Thefact .that two of the frequencies associated with station A are equal is an unusual case which isnot, in general, true of the other stations. This fact doe not afiect the operation of the system, and frequencies could have been chosen so that such condition would not exist. The important consideration is that no two of the stations transmit any signals of the same-frequency.

Station B transmits signals having the-following frequencies:

Station M transmits signals having the following frequencies:

The following three groups of signals with their corresponding values of K can be selected from the signals transmitted. from stations B and M:

Thus two signals having the same frequency can be obtained from each group of signals and their phase difierences obtained by a phasemeter. Three elementary systems having characteristic frequencies of 30,900; 3000 and 300 cycles are 18 thus provided, the :three "elementary systems forming a multiple system.

Similarly, three groups of four currents providing a multiple system made up of three elementary systems with characteristic frequencies of 30,009 .3000 and 39.0 cycles can be obtained from the signals-transmittedfrom any two transmitting stations in the combined system being described. Thus any three or more-transmitting stations form a complete system free from ambiguity. The range of frequencies in this combined system of 25 transmitting stations is from 11,970 to 18,030 cycles or .a band width of 6660' cycles, the frequencies being spaced 60 cycles apart so that approximately 100, frequencies are involved.

Fig. l5 a block diagram showing the mobile receiver suitable for use in the present combined system. As will presently be seen this receiver may be adjusted to receive the signals from any two of the. transmitting stations which will be supposed, by way of example, to be station 13 and Station M.

The, receiver shown Fig. .15 includes an aerial R and a frequency operator 6.8 having an amplifying filter 69 passing a band ranging from 10 to 20 kilocycles. It passes all of the frequencies of the signals transmitted from all of the transmitting stations including stations B and M. It also has two tunable oscillators 10 .and H. The oscillator 10 delivers a heterodyne frequency to the mixer 1'2 and also to the mixers 13,, "Hi and 75 through the phase shifters '88, 89 and 90 respectively. When signals are being received from stations 13 and M, the heterodyne frequency from the oscillator I0 is H'='84,'735 cycles. Under these conditions, the oscillator '1 I delivers a heterodyne frequency H"=83,405 cycles to the 'mixer '16 .and the signals from the amplifying filter 69 are also delivered to all .of the mixers 12 to 16 inclusive. The various mixers produce, among others, the following frequencies:

Theontputs of the. mixers 12 to 15 inclusive-arc delivered to the subtraction filters H to respectively, and the output of the mixer 7 6 is delivered to all of thesesubtraction filters. Subtraction; filter 737 .has a band pass of 100,000il5 cycles and selectsand mixes F1" and F1, i. e., the frequencies 1903105 and. 99,995 cycles, to produce a difierence frequency of 10 cycles. This is delivered. to and passed by the amplifying filter 81-. Subtraction filter "H3 has a bandpass of 79990::15 cycles and selects and mixes F2 and F2, e ,thefreqnenciesflflgfilfi and 69,995 cycles, to producev a difienencefrequency of 10 cycles which isdelivered to and passed by theampl ifymg filter as. Subtraction filter 19 has a band pass of 97-,G00i1'5 cycles and selects and mixes F3 and Ea i; e.,. the frequencies 97,005 and 96,995 cycles, to produce a difference frequency ofl-fl cycles which is delivered to and passedby the amplifying filter as. Subtraction filter so has a band pass of 99300215 cycles and selects and mixes Fi" and F4", i. e., the frequencies 99,705

19 and 99,695, to produce a difference frequency of 10 cycles which is delivered to and passed by amplifying filter 84.

The 10-cycle output current of the amplifying filter 8| is delivered to all of the servomotors 85 to 81 inclusive and the 10-cycle output currents of the amplifying filters 82, 83 and 84 are delivered to the servomotors 85, 86 and 81 respectively. These servomotors are mechanically connected by mechanical connections 9|, 92 and 93 to the phase shifters 88, 89 and 90 respectively so as to shift the phase of the heterodyne frequency passed therethrough from the oscillator 10. The servomotors and phase shifters each take a position such that there is zero phase difference between the two currents supplied to each of the servomotors and these positions are indicated on the dials 9P 92 and 93 respectively.

The indications of the dials SW 92 and 93 are respectively the phase differences between the two currents of equal frequency (10 cycles) derived from each of the three groups of frequencies Gem, GBMZ and GBM3 discussed above. The following relations can be derived:

where DM and DB are the distances in kilometers between the aerial of the mobile receiver and the aerials of the stations D and B respectively. It follows that the dials 9I 92 and 93 can be calibrated in kilometers, tens of kilometers and hundreds of kilometers, or in any other distance units, and the distances can be read directly without ambiguity.

The receiver of Fig. 15 can be tuned to similar groups of frequencies from any two of the transmitting stations A to Z inclusive by adjusting the oscillators 70 and II. Any two transmitting stations of the combined system constitute a multiple system. By employing two receivers or tuning a single receiver successively to groups of frequencies from two or more pairs of transmitting stations, a complete system giving a definite location for the mobile receiver is provided.

A block diagram of a suitable transmitting station is shown in Fig. 16. For convenience this will be assumed to be station B of Fig. 17. This station may include a transmitter 91 having an aerial B. It may also include an independent frequency generator 98 and three synchronizing generators 99, I and IOI, all of the generators feeding a single amplifier I02, in turn feeding the aerial B. The transmitting station may also include a control receiver having an aerial RB of fixed location and a frequency operator 68 which may be the same as the frequency operator 68 of the receiver of Fig. 15 except that the phase shifters are manually set instead of being mechanically driven by the servomotors. In this case, the servomotors 85, 86 and 81 of Fig. 15 form part of the synchronizing generators 99, I00 and IOI, respectively, of Fig. 16. Assuming that the station B is to be regulated from station M, the phase shifters of the frequency operator 68 of Fig. 16 are set and locked so that their dials I04, I and I06 indicate the distance DMDB above discussed, where DM and DB now refer to the distances between the aerial RB of the fixed receiver and the aerials of the stations B and M. The -cyc1e currents from the amplifying filters 8|, 82, 83 and 84 of the frequency operator 68 are delivered to the servomotor of the synchronizing generators 99, I00 and IDI through lines I01 to regulate these generators.

The frequency and phase of each of the frequencies F152, F133 and F 4 can thus be regulated to maintain the frequency of the respective pairs of currents from the frequency operator 68 equal and their phase difference zero. Under these conditions, any mobile receiver tuned to these two stations will indicate the correct distances Application to short-range navigation The combined system shown in Figs. 18 to 20 inclusive may include 50 stationary transmitting stations A to Z inclusive and a to z inclusive. In the map shown in Fig. 20, suitable positions of several of such stations are indicated. Each station transmits signals of six frequencies. As in the case of the long-range system of Figs. 15 to 17, the present system is based upon the selection of basic frequencies, in this case, six basic frequencies (F1 to F6 inclusive) which bear a 1 definite relation to each other so as to give in the system indications of five degrees of accuracy, preferably differing from each other by a factor of 10. As a specific example, the following basic frequencies have been chosen, although other basic frequencies can be employed.

F1=1,615,555 Fz=1,634,445 F3=1,904,805 F4=1,908,055 F5=1,940,230 F6=1,940,555

These frequencies were chosen so that the following relations are satisfied:

The frequencies FAl to FAB transmitted by station A are derived from the basic frequencies F1 to Fe respectively in accordance with the following relations, where ha. is a constant:

The frequencies transmitted by any other station are found by merely substituting the subscript appropriate to that station for the As 'in the above relations. Thus, for example,

21 The constants f-A' tO fz and fa to fzwere chosen in accordance with the following relations:

Sincez-there are a'totalof50 transmitting stations, .each transmitting six frequencies, the system .employs a total of 300 frequencies. With the frequencies F1 to Fe above assumed, the frequencies are distributed :in three bands about 20 kilocycles .wide, the central frequencies of the variousibandsibeing 1625, .1915 and l950'kilooycles.

.Fromthe relationsabove given, the frequencies of the signals transmitted from stations .Aand a are .as follows:

'F 'l,6l'5,55 +7l5= 1,616,270. 0. F42: 1,634,445- 715; 1,633,730. 1. F 1,904,805+715=1,905,520 c.

F111=,l,908,055+7l5= 1,908,770 0.

1 1,, 1,615,555 65= 1,616,220 0. a2 =-1,634,445 665 1,633,780 0. F,,1.- -1,9011,805+665= 1,905,470 0. 2, =.1 ,908,-055 +665: 1,908,720 0. F, =1,940,230+565=1,940,895 c.

Eromtheabove frequencies, fivegroups having four frequencies each may be selected which satisfy the relation These groups of frequencies with .theircorrespending values .of K are as follows:

' Group G111of frequencies F1 1, F112, F111 and E212 having values of K of +1, +1, 1 and 1 re- .spectively;

' group .GAZ of frequencies FAG, F111, FssandnFa having values of K of +1, 1, -1 and +1 res e ely;

Group GAB of frequencies FAG, F114, Fas and F11 haying .values of K of +1, 1, 1 and +1 respectively;

Group G of frequencies F111, F113, F114 and F113 having values of K of +1, 1, -l and +1 respect e y;

group G115 of the frequencies F115, F115, F116 and Fas having values of K of +1, l, .1 and +1 r spect y- Fig. 18 shows a suitable receiver for employ- ;ment in .the present system. This receiver in- .cludes ,a frequency operator I08 and five phase- .meters I09 to H3 inclusive. For purposes of explanation, it W ll be assumed that this receiver is tuned to receive the signals from transmitting ;stations A and 11, although it may be tuned to receive the signals from any other two stations assigned the same letters of the alphabet in the present explanation, such as B and b, C and 0, etc.-

The signals from the various stations including stations ,A and a are received by the aerial R and delivered to the amplifying filters I2I, I22 and .123. The amp fyin .filte ba band pass of 1625i12 kilocycles and thus passes the 22 frequenciesFsnFAz,F11 and 'fFa2. The amplifying filter 1 22has-a band pass of 1915-.Ll3 kilocycles and thus passes frequencies F113, F111, F113 and Fad. The amplifying filter I2 3 has a band pass of 1950:12 kilocycles and thus passesfrequencies F115, FAG, Fa5 and Fafi. The outputs of amplifying filters I2I, ,IZZand I2-3-are delivered to the mixers H4, H5 and 1.16 respectively and at the same time a carefully stabilized heterodyne frequency H1 of 1791 kilocycles isdelivered to these mixers. The outputs of these mixers are delivered to the amplifying filters I24, I and I26 respectively. Since --the latter amplifying filters have band passes of 165:12 kilocycles, 124:13 kilocycles and 1-59: 12li1ocycles respectively, they pass-only the difference frequencies corresponding to the received signals which areas follows:

Amplifying filter I24 F115.H1=l49,895 cycles Fa6-H 1=150,220 cycles The outputs of the amplifying filters are delivered to mixers ,I I1, H8 and I19 respectively cycles cycles cycles cycles cycles cycles and an adjustable heterodyne frequency Ha from The first and third of these frequencies are The output :of this MI. The output of this mixercontains thedifference frequencies 21,270 and 21,220 cycles, which are selected and mixed by the subtraction filter I 33 to produce difference frequency -of cycles. The other two frequencies listed above from the mixer -I I. 1,nam ely, the frequencies of 24,270 and 24,220 cycles, are selected by and mixed in the subtraction-filter I28 to produce diiferencefre- .quency .of 50 cycles.

.outputof mixer I IScontains the following difference frequencies:

The first and third of these frequencies are selected .by and mixed in the subtraction filter separate 50-cycle currents.

I36 to produce a difference frequency of 50 cycles. The second and fourth frequencies are selected by and mixed in the subtraction filter I31 to also produce a difference frequency of 50 cycles.

The output of mixer I I9 contains the following difference frequencies:

The subtraction filter I38 selects and mixes the first and third of these frequencies to produce a difference frequency of 50 cycles. The subtraction filter I39 selects and mixes the second and fourth of these frequencies to also produce a difference frequency of 50 cycles.

The outputs of subtraction filters I33 and I20 to I32 inclusive thus each contain an output frequency of 50 cycles derived from the 12 signals of different frequencies from the stations A and a. The outputs of these subtraction filters are delivered to the amplifying filters I34 to I39 respectively, and these latter filters select and pass the 50 cycle difference frequencies to provide six The current from the amplifying filter I34 is delivered to phasemeters I09 and I10. The current from amplifying filter I35 is delivered to phasemeter I09.

The current from amplifying filter I35 is delivered to phasemeter I I2. The current from amplifying filter I3! is delivered to phasemeters III and H2. The current from amplifying filter I38 is delivered to phasemeter I I3 and the current from amplifying filter I39 is delivered to phasemeters Where (p is in degrees and the distances DA and D5. are in kilometers and represent the distances between the aerial of the mobile receiver and the aerials of the stations A and a respectively. The receiver of Fig. 18 may be tuned to any other pair of transmitters B and b, C and 0, etc. by adjusting the frequency of oscillator I42. By employing two receivers tuned to different pairs of stations or by successively tuning a single receiver to two different pairs of stations, a complete system made up of two multiple systems each containing five elementary systems is obtained and the mobile receiver may be located without ambiguity.

Fig. 19 shows a pair of transmitting stations suitable for employment with the receiver of Fig. 18 in a short-range system. These stations may, for example, be the transmitting stations A and a. Station A may include a transmitter I43 provided with six frequency generators I44 to I49 inclusive. All of these generators may have their frequencies independently controlled, for example, by crystal-control of their oscillators. All of the generators feed a common amplifier I50 which in turn feeds the transmitting aerial. Station a, similarly, has six generators I52 to I51 inclusive, five of which (I52 to I56 inclusive) are synchronizing generators and one of which (I51) may have its frequency independently controlled.

As in station A, the generators all feed a common amplifier I58, which in turn feeds the transmitting aerial.

Station (1 also includes a control receiving station, preferably located a few kilometers therefrom, having a receiving aerial RA of fixed location which receives the signals from both station A and station a. The received signals are delivered to a frequency operator I08 which may be identical with the frequency operator I08 of Fig. 18. This frequency operator produces six currents of equal frequency cycles in the example assumed) as described with reference to Fig. 18. These six currents may be delivered by lines I to the synchronizing generators I52 to I55 inclusive, each of which contains a servomotor mechanically connected to frequency and phase adjusting devices as described with refer-' ence to Figs. 2 and 3. The servo-motors in the synchronizing generators I52 to I55 inclusive may be connected to the output of the frequency operator I08 in the same manner that the phasemeters I09 to I I3 are connected to the output of the frequency operator I08 in Fig. 18. Five manually adjustable phase shifters I6I to I inclusive may be interposed in five of the lines I60. These phase shifters may be employed to adjust the phases of the five currents of equal frequency to compensate for the distances between the fixed receiving aerial and the transmitting aerials and for any other unavoidable phase shifts in the system. That is to say, the transmitting station a is regulated to cause each of the groups (Gm to GAS inclusive) of four frequencies mentioned above and associated with the stations A and a to satisfy at all times the relation This regulation will also maintain the constant at any desired value or zero at a selected point in the system.

It is apparent that the system of Figs. 18 to 20 is made up of a plurality of multiple systems, each having five elementary systems, and that two space-transmitting stations are employed for each multiple system. Each multiple system employing. two transmitting stations is independent of the others except for the selection of frequencies, as above discussed, which avoids the employment of two signals of equal frequency by different multiple systems in the same area. In many cases, two or more stations will be positioned at the same location as shown in Fig. 20, in which case the same aerials as well as other elements of the transmitting stations may many times be employed for more than one station.

It will be further apparent that any of the combined systems described need not employ all of the stations provided for. Thus the number of stations in the combined systems of Figs. 15 to 1'7 need not be as great as 25, and the number of stations in the combined system of Figs. 18 to 20 need not be as great as 50.

It was assumed in the above description that all the currents which are used are sinusoidal currents. It is to be noted that a current which is modulated periodically in any manner may be demodulated to provide a plurality of sinusoidal currents. Modulated signals can, however, be employed in the present invention, it being understood that unwanted frequencies can be eliminated either at the transmission stations or at the receiving stations.

Likewise, it was assumed, that all the waves which are used are radio-electric waves. It is to be noted that a sinusoidal electric current may be transformed by means of suitable radiators int-'0 a compression wave of the same frequency (ultra-sound, for example), or into a light-wave modulated through the current and that the: so obtained compression or light-wave may be retransformed by means of a suitable device into an electric current havin the same frequency as thecurrent having given rise tosaid wave. The

invention may be applied, therefore, to systems which use such waves.

Having now described our invention what we claimas new and desire to secure by Letters Patent is:

I. In a system for determining the location of a receiving pointrespective to spaced transmitters by comparing at said receiving point the phases "of signals emitted by said. transmitters and. re-

ceived at said receiving point,attleast two spaced transmitters for emitting: at least three different high frequency waves; the choice of said. frequencies being limited bythe only condition which issufiici'ent and necessary that the algebraic sum of the-respective products of. said frequencies by any integers, at least one of which. is positive and at least: one of which negative, is zero:,. at said receiving point: a receiver comprising in combination filtering; amplifying and mixing meansto receive said waves and. deduce therefrom. by means of differing frequency mixtures two currents. having: the same frequency and 'phasemeteringrmeans-tomeasure the phase difference betweensaid two currents; thereby to provide an indication about the location of. said receiving. point respective to said transmitters.

2. In a system for determining the location ot a. receiving point respective: to spaced transmi'tters by-c'omparing' at said receiving point. the phases of signals emitted: by said transmitters and received at said: receiving point, at least two spaced transmitters for emitting at. least three difterent-high frequency waves, the choice of said frequencies being limited by the only condition whichxare sufficient and necessary that the algebraicsumrofi the respective: products of said frequencies by any integers, at least one of which is positive and: atleast one of which is negative,

at least substantially zero; means for canceiling said algebraic sum and for automatically maintaining it: at zero, saidmeans comprising in combination: a receiver comprising incombination filtering, amplifying andmixing means, to receive said waves and todeduce therefrom by means. of frequency mixtures two currents the difference between the frequencies of. which. is equal to said. algebraic sumand: means responsive to the differencebetween the frequencies and the difference between the phases of said two currents to act upon the frequency and the phase of one of. saidemitted. waves to cancel said frequency difference and consequently to cancel said. algebrai'c sum, atsaid receiving point: a-receiver comprising in combination filtering, amplifying and means to receive said waves and deduce therefrom by means of differing frequency mixtures two currents having the same frequency and phasemetering means to measure the phase difierence between said two currents, thereby to provide: an indication about the location of said receiving point respective to said transmitters.

31ina system utilizing at least three signals the frequencies of which mustsatisfy the relation where F1, F2, F11 are said frequencies and K1, Kz, Kn are integers atleast one of which is positive and' at least one of which is negative, whereby two currents of equal frequency can be derivedfrom said signals by combination filtering amplifying and mixing means when said relation. is satisfied, generators for said signals, a device comprising in: combination filtering, amplifying and mixing means for deriving from said signals two currents having diiferent frequencies when said relation isnot satisfied, a two. phase asynchronous motor having a two-phasestator winding fed by said two currents derived by said device, one of said generatorshavingmechanical.- ly actuated frequency and phase control means for the signal generated thereby and mechanical means to connect the rotor of said two phase motor to said frequency and phase controiling means to actuate the same. and. canoet the frequency difference of said two currents derived by said device,

t. In. a system. for determining the locatiorrof a receiving p'ointrespecti've to spaced transmitters by comparing at said receiving point the phases of. signals emitted by said transmitters and received at said receiving point, at least two'spaced transmitters for emitting at least three difi'erent high frequency waves, the choice of :said frequenciesbeing limited by the only conditions which are necessary and. sufixcient that thealgebraic sum of the respective produ'cts'of said frequencies by any integers which. is sufficient and necessary is at least substantially zero, meansfor cancelling said algebraic sum and for automatically' maintaining it at zero, said means comprising in' combination: a receiver comprising in combination filtering, ainplifying and mixing means, to receive said waves and to deducether'efrom by means of frequency mixtures two' currents the. difference between the frequencies of which is equal to said algebraic sum, atwo phase motor" the stator of. which is fed by saidtwo currents, meansto actuponithe frequency 2111111188115 to: act upon the phase of one of said signals and mechanical means to connect the: rotor of said two phase motor respectively to said frequency controlling means and to said phase controlling means to cancel thefrequeneydifference of said twocurrents and consequently saidalgebraic sum, at said'receivingpoint': a receiver comprising in combination filtering, amplifying and mixing means to receive said waves and deduce therefrom. by means of differing frequency mixtures two. currents having the same frequency and phase metering means to measure" the phase difference between-said two currents, thereby to provide an. indication about the location of said receiving point respective to said transmitters.

5. In a system. for determining the location of a receiving point respective to spaced transmitters by comparing at said. receiving point the phases of signals emitted by said transmitters and received at said receiving point, a first, a second, a third, a fourth and a fifth transmitter to emit respectively five continuous pure, high frequency waves having respectively a first, a second, a third, a fourth and aifi'fth frequency, said first, second, fourth and fifth transmitter being respectively located at the four corners of a square in the center of. which is located the third transmitter, said frequencies being all comprised in one com paratively narrow band, said first frequency being smaller than said second frequency, which is smaller than the third, said third. frequency being smaller than the. fourth whichv is in turn smaller than the fifth, the respective gaps be- 

